The present invention relates to plasma polymerization and more particularly to a highly abrasion-resistant plasma coating.
The term plasma polymerization refers to the deposition of films of organic material by subjecting vapors of organic materials to a glow discharge. Plasmas typically are characterized by their electronic temperature and density. In a glow discharge process, the average electron energy ranges from about 1:10 eV with ne ranging from about 10.sup.9 -10.sup.12 cm.sup.-3. In such plasmas, a large difference in the electron temperature, T.sub.e, and the gas temperature, T.sub.p, is present. Typically, T.sub.3 is about 10 to 100 times larger than T.sub.p. Thus, it is possible to have a plasma in which the gas temperature is at room temperature while the electrons have sufficient energy to rupture molecular bonds and generate highly active species. Because of this, glow discharge plasmas also have been called "cold plasmas". Coating by plasma polymerization inherently is an ultra-thin film technology which heretofore has been practical only for those applications in which a very thin film is sufficient for obtaining the desired properties. Thin film limitations result from the low deposition rates typically encountered in plasma polymerization and because thick films tend to be unstable because of their extreme hardness and residual strains due to the high degree of cross-linking.
Prior proposals for producing abrasion-resistant plasma coatings include U.S. Pat. No. 4,435,476 which applies an organosiloxane coating liquid to a solid substrate followed by exposure of the coated substrate to glow discharge treatment under vacuum. U.S. Pat. No. 4,137,365 proposes to improve the abrasion resistance of a polymerized organosilane coating on a plastic substrate by treating the silane-coated substrate in an oxygen plasma. This same oxygen post-treatment technique also can be found in U.S. Pat. No. 4,018,945. U.S. Pat. No. 4,096,315 proposes to coat an optical plastic substrate by first exposing the substrate in an evacuated plasma polymerization chamber to a first plasma that forms hydroxyl groups on the substrate surface; then plasma coating said substrate with a silicon-containing monomer; and then exposing the coated substrate to a third plasma consisting of a noble gas, oxygen, nitrogen, or air. U.S. Pat. No. 4,492,733 forms a plasma polymerized coating from tri-functional silanol molecules followed by a post-plasma treatment in a vacuum.
Despite these and other proposals in the art, abrasion-resistance of plasma coatings still is a problem. For example, in coating polymeric substrates for use in optics, it is desirable to produce a plasma-applied film which possesses non-glare characteristics and an abrasion resistance of less than about 5% optical loss after 100 cycles in a Taber abrasion resistance test as described in ASTM D4060-81.